An unusual species of turtle has been found to excrete waste substances through its mouth, according to a team of scientists in Singapore.

The scientists were puzzled by the behaviour of the Chinese softshell turtle (Pelodiscus sinensis) as, despite needing to breathe air, it often submerged its head in water for up to 100 minutes at a time.

When they studied the turtle in the lab, the team found that it regularly dipped its head into water and rinsed it through its mouth. The rhythmic motion of its throat, not to mention the fact that it did not drown, indicated that it was still ‘breathing’ while submerged.

Excreting urea

After testing the water, the scientists found increased levels of the chemical compound urea, a nitrogen-rich waste substance that is excreted by most vertebrates via the kidneys and passed out as urine. In turtles, urea normally passes out of the cloaca, a single orifice used for excretion and for reproduction.

However, the team’s findings showed that the Chinese softshell turtle excretes significantly more urea through its mouth than through its cloaca. This adds to previous research that indicated that this species has highly specialised mouth tissues, a fact first discussed over a century ago when it was suggested that their velvety mouth functions in a similar way to fish gills. The findings of the research have been published in The Journal of Experimental Biology.

According to Professor Ip Yuen Kwong, one of the researchers, “These results indicate for the first time that [mouth tissue] processes and rhythmic [throat] movements were involved in urea excretion in P. sinensis.”

“We were greatly surprised by our novel results because it is generally accepted that the kidney is responsible for the excretion of urea in vertebrates – except fish,” he said.

The Chinese softshell turtle is typically found in swampy, brackish water, and the scientists have suggested that the ability to excrete urea via the mouth may have helped this and other soft-shelled turtles to successfully invade brackish and marine environments.

To produce urine in the kidneys, the turtles would have to regularly take in water, which would be harmful when the water is too salty. By simply rinsing its mouth with the brackish water, the turtle can avoid the problems associated with drinking it.

The Chinese softshell turtle may also be able to take in oxygen through its mouth tissues.

Considered a delicacy in many parts of Asia, this species is farmed in vast numbers for food, but its wild populations also continue to be exploited. As a result, the Chinese softshell turtle is in decline, and has been classified as Vulnerable on the IUCN Red List.